Phase change memory devices, such as phase change RAM (Random Access Memory), use as nonvolatile data a difference in electrical resistivity due to a reversible phase change between amorphous and crystalline in a phase change material of chalcogenide series. Such phase change memory devices have been suggested by Ovonyx, Inc. and ECD (Energy Conversion Devices), Inc. in U.S.A. (see U.S. Pat. Nos. 5,359,205, 5,414,271, 5,536,947, and 5,534,712).
A writing operation of a memory, which is also referred to as “reset”, changes the phase of a phase change material in a phase change memory device to an amorphous state. During the writing operation, the phase change material is heated to a melting point or more using Joule heat produced by an electrical pulse, and is then rapidly quenched. In this way, the phase change material in the phase change memory device is changed to the amorphous state. This state is called a reset state, in which electrical resistance is high.
An erasure operation of a memory, which is also referred to as “set”, changes the phase of the phase change material to a crystalline state. During the erasure operation, thermal energy is supplied to the phase change material in accordance with Joule heat produced by an electrical pulse, and the phase change material is heated to a crystallization temperature or more and then maintained for a predetermined time or more. In this way, the phase change material in the amorphous state is changed to a crystalline state. This state is called “set state”, in which electrical resistance is low.
A reading operation of a memory is performed by reading resistance of the phase change memory device or a change in voltage across a bit line.
Similarly to other semiconductor memory devices, the phase change memory device has a limited writing life time. At the beginning of development, it has been reported that the writing life time of the phase change memory device is about 1013 (see S. R. Ovshinsky, Tutorials in 2003 Material Research Society Fall Meeting).
FIG. 1 shows a test result of cycling endurance of a phase change memory device (see Stefan Lai, IEDM Tech. Dig., 2003, pp. 255-258).
Advancement of device integration causes a decrease in cycling endurance of the phase change memory device. A latest report shows that the cycling endurance of the phase change memory is about 105, which is much the same as a flash memory (see J. H. Oh et. al., in IEDM Tech. Dig., 2006, 346905).
In regard to a phase change memory device that is put into an unwritable state due to repetition of the writing and erasure operations, an analysis result shows that the unwritable state is caused by a change in composition of the phase change material (see Jong-Bong Park et. al., Jour. Electrochem. Soc. 154, H139). The writing and erasure operations of the phase change memory device are accompanied by heat and high current. Heat and high current lead to thermal atomic diffusion or collision against electrons at high temperature, resulting in electromigration. The electromigration causes the movement of atoms of the phase change material, resulting in a change in composition of the phase change material.